Spectroscopic studies of the intracluster hydration reaction of NO₂⁺

Infrared spectra of NO₂⁺(H₂O)_n clusters formed in a pulsed discharge source were recorded by vibrational predissociation spectroscopy as a function of cluster size. A significant change in the vibrational spectra and the predissociation product yields were observed at n = 4, suggesting the onset of a rearrangement at this critical cluster size. For NO₂⁺(H₂O)_n with n ≤ 3, the clusters possessed red-shifted H₂O bands and dissociated primarily by loss of H₂O. These were clusters of H₂O ligands bound to a nitronium ion core. At n = 4, we found evidence for the occurrence of an intracluster hydration reaction NO₂⁺ + 2H₂O → HNO₃ + H₃O⁺. Vibrational excitation of the cluster led to loss of HNO₃ as the major channel, as well as loss of H₂O. New vibrational bands appeared and were assigned to solvated H₃O⁺ and to HNO₃ ligands. With the H₃O⁺ replacing NO₂⁺ as the charge center, the clusters rearranged to form H₃O⁺(H₂O)₂(HNO₃). Upon addition of a fifth water molecule, the hydronium ion was further stabilized by completion of the first hydration shell, leaving HNO₃ in the second shell.